Implant for Osseous Anchoring with Polyaxial Head

ABSTRACT

An osseous anchorage implant comprising an attachment structure adapted to receive and be attached to at least one bar, that is suitably an element of an osteosynthesis. The implant is made up of an osseous anchor and an attachment head bearing the attachment means. The attachment head is suitably traversed by a channel that is adapted to receive the bar. The attachment head comprises a clamp that is adapted to clamp the bar against an inside support wall of the channel. The implant is further characterized by the attachment means and the support wall enabling the rotation of the bar about a first axis that is not parallel to the longitudinal axis of the bar. The clamp comprises a moving clamping face, that is adapted to contact the bar, and that is borne by a support head that is articulatable at the end of the clamp via a ball and socket connection. The channel is suitably an open channel having an aperture opening onto one of the lateral faces of the attachment head, wherein the aperture bears the clamp. The bar is enabled to be introduced into the channel from a side of the channel.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/498,234, filed Dec. 7, 2004, which is a U.S.C. § 371 application ofPCT/IB02/05302, filed Dec. 12, 2002, which claims priority to FR01/16122, filed Dec. 12, 2001. All of the foregoing are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an implant providing osseous anchorage,for example in a vertebra for anchoring an osteosynthesis device. Thisimplant comprises a head capable of receiving a bar linking a pluralityof implants in different angular position.

BACKGROUND

It is known to fix one or a plurality of implants into one or aplurality of osseous elements in order to connect to the skeleton adevice implanted in the human or animal body, said implants being thenused to fasten certain elements of said device. For maintaining orcorrecting the rachis in particular, use of an osteosynthesis devicecomprising one or a plurality of maintenance bars or plates positionedalong the vertebral column and fixed to certain vertebrae by implants iswell-known. These implants are fixed on the one hand to the bars and onthe other to the vertebrae by an osseous anchoring means comprised of ahook having its support on a vertebra or of a threaded part screwedinside the vertebra, for example, at the pedicle. In the case of anosteosynthesis of the extreme vertebrae one or a plurality of implantscan of course be securely fastened to adjacent bone, the sacrum forexample.

The FR 0104717 patent discloses for this purpose an implant comprisingan osseous anchoring part and a fixation head traversed by a channelwhere the bar is clamped. This document describes an implant, whose headcomprises a lateral opening enabling the introduction of a bar with flatareas wherein a clamping screw has already been pre-installed in thesuperior part of said head. In order to assure satisfactory contact ofthe clamping screw on the flat surface whatever the angular position ofsaid flat surface about the axis of the pin, the clamping screw isprovided with a tiltable support surface mounted on a ball and socketjoint.

By supporting itself on the inside shape of the channel under the effectof the clamping, the bar's position is thus definitively determined bythe general position of the implant. At the time of attachment of thebar to an implant that is already anchored in the rachis, if the bar isnot in a position corresponding to that of the channel, the act offorcing its introduction can induce a certain stress in the bar.

On the one hand, said stress can then make the attachment of the bar tothe implant difficult or render its clamping not very accurate. Thisproblem is true more particularly when the bar has already been engagedwith a first implant and is being attached to a second implant. In orderto reduce this stress, it is conceivable to anchor the implant accordingto the position of the bar, but this is not always possible to do noreasy to predict. It would also be conceivable to deform the bar, whichcould be a problem in the situation and plays against the requirement ofa rigid bar to assure an effective hold. This stress can thus makemanipulation delicate to execute, in particular in weale intrusionsurgical procedures, for example video-assisted or laparoscopicprocedures.

On the other hand, even if clamping enables stressing the bar to adopt aposition or a shape corresponding to the implant, said stress willpersist permanently over a very long time after the procedure. The factthat the bar is under permanent stress poses the risk of a mechanicaleffect directly on the rachis, for example causing pains or changing ordisturbing the correction or the support sought by using theosteosynthesis device.

SUMMARY

An object of the present invention is to eliminate at least one drawbackof the prior art by providing an osseous anchorage implant capable ofadapting itself, at least to a certain extent, to different orientationsof the bar when the implant is already anchored in the osseous element.

This object is achieved by an implant for osseous anchoring comprising afixation structure that is capable of receiving and fixing at least onebar, in particular a bar of an osteosynthesis device. The implantcomprises on the one hand an osseous anchoring structure, and on theother hand a fixation head bearing the fixation means. The fixation headis traversed by at least one channel that is adapted to receive the bar.The structure further comprises a clamp adapted to clamp the bar againstone inside wall of the channel, called a support wall. The implant ischaracterized in that the fixation structure and the support wall enableobtaining, prior to locking the fixation structure in place, apre-determined clearance in rotation around at least one first axis thatis not parallel to the longitudinal axis of the bar. The clamp comprisesa face that is adapted to be in contact with the bar, called a movingclamping face. The moving clamping face of the clamp is borne by asupport head articulated at the end of the clamp by means of a ball andsocket connection. The channel has the form of an open channel having anaperture opening onto one of the lateral faces of the fixation head,wherein one edge of said aperture bears the clamp. The-aperture of thechannel and the position of the clamp thus enable introduction of thebar by a lateral route.

According to another feature, the fixation structure and said supportwall enable, prior to blocking the fixation structure, a pre-determinedclearance in rotation around at least one second axis not parallel tothe first axis and not parallel to the longitudinal axis of the bar.

According to another feature, the channel opens at each side of thefixation head through apertures whose disposition and dimensions enablethe bar to have a pre-determined clearance inside the channel inrotation at least about one axis that is substantially perpendicular tothe axis of the channel and to the support direction of the clampingmeans.

According to another feature, the support wall comprises an element,called a moving baseplate, having a clearance in rotation at least aboutone axis not parallel to the axis of the channel and to the supportdirection of the clamping means on the bar, the moving clamping facehaving a determined clearance in rotation around one axis substantiallyparallel to the axis of rotation of the moving baseplate.

According to another feature, the moving baseplate has a part in theform of a spherical portion supported by means of a complementarycontact in a housing formed in the fixation head.

According to another feature, the moving baseplate is in contact withthe bar through a support face having a shape that is substantiallycomplementary to the exterior shape of the bar. The moving baseplatecomprises at least one formal irregularity that is adapted to co-operatewith a formal irregularity borne by the fixation head to form a stopthat is adapted to hold the support face of the baseplate turned towardsthe side of the bar.

According to another feature, the formal irregularity is a pin disposedon the underside, the rear or on the front of the moving baseplate,adapted to cooperate with a cavity with a greater dimension that isborne by the fixation head.

According to another feature, the formal irregularity is a cylindricalhole, preferably drilled, disposed on the front of the moving baseplateand adapted to cooperate with the cylindrical head of a pin held in holeformed in the fixation head.

According to another feature, the clamp comprises a clamping screwinserted into a hole that is traversing to one edge of the channelopening and is adapted to cooperate with said drilled hole to produce asupport for the bar.

According to another feature, the osseous anchor comprise either athreaded part capable of anchoring in an osseous element by co-operationwith the osseous material of said osseous element, or a protruding partcapable of anchoring at the surface of an osseous element byco-operating with at least one formal irregularity of said same surface.

According to another feature, the osseous anchor comprises an elongatedand threaded part, the axis of the channel being substantiallyperpendicular to the longitudinal axis of the osseous anchor.

According to another feature, the channel has a shape comprising twohead to tail truncated cones with their minor baseplates facing eachother and joined to each other directly or by means of a cylindricalpart.

According to another feature, the bar bears one or a plurality of flatareas on its external surface, whereby providing a flat contact surfacefor the support head.

The invention together with its features and advantages will be clearerby reading the description thereafter with reference to the annexeddrawings, wherein:

DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of an implant according to this invention;

FIG. 2 is a side elevation of an implant according to this invention;

FIG. 3 is a top perspective view of an implant according to thisinvention;

FIGS. 1, 2, and 3 together represent an implant with a moveable head andosseous anchorage;

FIG. 4 is a sectioned partial side view of an implant with a fixed headduring introduction of the bar, shown in section transverse to the bar;

FIGS. 5 and 6 are perspective views of an implant with a fixed head, abar and an osseous anchorage through screw with portions cut away alongthe axis of the bar, and transverse to the bar, respectively;

FIG. 7 is a perspective view of an implant that has osseous anchorage bymeans of a hook and a fixed head in longitudinal section along the axisof the bar and shown without its clamp;

FIGS. 8 and 9 are, respectively, a perspective view and a sectional viewof an implant with a fixed head according to a second embodiment of theinvention;

FIGS. 10 and 11 are, respectively, a perspective view and a sectionalview of an implant with a fixed head according to a third embodiment ofthis invention;

FIGS. 12 and 13 are, respectively, a perspective view and a sectionalview of an implant with a fixed head according to a fourth embodiment ofthis invention.

DETAILED DESCRIPTION

The invention described herein relates to an implant comprising on theone hand a means for holding a bar and on the other hand means foranchoring it to an osseous element.

In a general fashion, in the field of surgery, an implant is defined asan object intended for being implanted in a human or animal body and toremain there continuously after the surgical procedure, at least over acertain period of time. More precisely, one speaks of a prosthesis todesignate a device realizing a function, for example a movement or anarticulation. Although not comprising a prosthesis per se, it must beunderstood that the implant described herein can comprise part of aprosthesis device or can be used for attachment of such a device to astructural part of a patient.

By way of example, an implant according to the invention is describedherein in the form of an implant incorporated in an osteosynthesisdevice, such as used to hold, prop, or straighten the rachis. Thisfunction is thus assured by one or a plurality of rigid bars or similarelements, connecting a plurality of rachis elements, such as the sacrum,the vertebrae or parts of vertebrae with each other. In the same sense,such a bar can obviously also be used also to connect another implant orprosthesis to a skeleton such as, for example, an artificial vertebra,an arthrodesis frame, or an intervertebral disc prosthesis.

In its part affixed to an osseous element, said bar is functionallyattached to an osseous element by means of one or a plurality ofimplants comprising an osseous anchor such as, for example, a screw orone or a plurality of hooks. The implants thus comprise fixationelements that are adapted to receive-the bar before, or in the courseof, a surgical procedure. The-bar is then attached to the implant.

In one embodiment represented in FIGS. 1 to 6, the anchor comprises anelongated part having a thread that is capable of being screwed into theosseous material, for example at the pedicle or body of a vertebra orinto the sacrum.

In one embodiment represented in FIG. 7, said osseous anchor comprises ahook that is adapted to engage a formal irregularity in the osseouselement, such as a pedicle or a vertebral or sacral protrusion, or atransverse apophysis.

In the course of the surgical procedure, when an implant is anchored inan osseous element and a bar is to be fixed to it, the position in whichthe bar can be introduced does not always correspond to that which wouldbe the easiest to assemble to the fixation structure of the implant.Said position can be restricted, for example, by the anatomicalenvironment or by the fact that said bar is already attached to anotherimplant.

Moreover, when the bar is inserted into the fixation structure, if thelatter is not properly aligned with the axis of the bar, themisalignment of the contact or fixation surfaces can be the cause ofpoor fixation, such that the attachment is not-sufficiently rigid orsecure.

In particular, if the bar must be forced at the time of the attachmentto be adapted to be mated with the relative position of the implants,said force can result in residual stresses in the structure of theosteosynthesis device. Such persisting stresses can consequently impairthe patient's daily comfort or perturb or change the desired effect ofthe device.

In order that the attachment element can be better accommodated to theposition of the bar, the attachment element according to the inventioncomprises at least one element having a certain movability. The positionof the moveable element can be adjusted by rotating it around one or aplurality of axes that are not coincident with the longitudinal axis ofthe bar and, for example, perpendicular to said longitudinal axis of thebar. Said rotation can be executed within a pre-determined clearancethat is capable of enabling maximum angular positions or according topredetermined angular positions or capable of not being limited, that iscompletely free of constraints. According to the embodiments, saidattachment structure can comprise an element adapted to move along oneor a plurality of axes or it may comprise a plurality of elements whichare themselves adapted to move along one or a plurality of axes.

The bar can thus be assembled in a plurality of angular positionsrelative to the anchor or to the osseous element. These variations inangular position can particularly comprise an adjustable tilt relativeto the osseous surface to which the implant is anchored or a rotationaround an axis extending from said osseous surface or a combination ofthe two.

In the embodiments represented herein, the implant (1) (see FIG. 5) isrigidly connected to the bar (2) by an attachment structure comprising achannel (12) disposed in one part of the implant, said part beingdesignated as the fixation head (11). The channel (12) can have theshape of a channel that is open at its two ends and open on one of itssides. Such a lateral opening (120) thus enables introduction (see FIG.4) of the bar (2) through the side of the channel without the necessityof having to thread the bar through an end.

Once inserted into the channel (12), attachment of the bar is assured bya clamp (4) that is supported by means of at least one side of said barso as to make contact with the bar and force it against a wall, calledthe support wall, of the channel (12). Said clamp comprises, forexample, a clamping screw (41) mounted in a drilled hole (14) borne byone part of the fixation head (11) constituting one edge (124) of thelateral opening (120) of the channel (12). Said clamping screw (41) hasan external threading that co-operates with an internal threading ofsaid tapped hole (14) to move the screw (41) along a clamping axis (d4)and bring it into contact against the bar (2), thus clamping theclamping screw (41) to comprise formal irregularities, for example aninternal mark, enabling the use of a clamping tool to achieve forcingthe bar (2) against a wall. The bar can advantageously have one or aplurality of flat areas upon its external surface in order to enable aflat contact surface to be obtained with the clamping screw (41) andthus the reliability of forcing the bar against the wall is enhancedcompared to a punctal or linear contact.

In one embodiment represented in FIGS. 4 to 13, the channel (12) has asupport wall having an element, called a moving baseplate (3), that ismoveable relative to the fixation head (11). Said moveable baseplate (3)has a part (31) suitably in form of a spherical surface positioned in acomplementary contact in a housing formed in the wall of the channel(12). By virtue of said spherical contact, the moving baseplate (3) hasa certain freedom of rotation around the center of its complementaryspherical part (31). Said moving baseplate has in particular a certainclearance (a3) in rotation around an axis (d3) that is substantiallyperpendicular to the longitudinal axis (d12) of the channel and to thedirection of support of the clamping means. On its face, that is thesupport face (32), in contact with the bar, the moving baseplate has ashape complementary to the external surface of said bar, for example inthe form of a cylindrical portion, that provides a good contact surfacewhen clamped.

On its part (31) in the form of a spherical surface, the movingbaseplate (3) can have one or a plurality of formal irregularities (310)that are adapted to cooperate with one or a plurality of formalirregularities of the housing of the fixation head (11) to form a stoplimiting the clearance in rotation of the moving baseplate. Said formalirregularities (310) can be, for example, a pin protruding from themoving baseplate and co-operating with a larger dimensioned cavityformed on the complementary contact surface. Said stop, for example,tends to prevent excessive turning of the moving baseplate and assuresthat it properly presents its support facing the bar. Thus, in theembodiment represented in FIGS. 4 to 7, the formal irregularity (310) isa pin disposed under the spherical part (31) of the moving baseplate(3), and in the embodiment represented in FIGS. 8 and 9, the formalirregularity (310) is a pin disposed on the rear of the moveablebaseplate (3). Likewise, in the embodiment represented in FIGS. 12 and13, the formal irregularity (310) is a pin, having an oblongcross-section, disposed on the front of the moving baseplate. Thedimensions of the cavities adapted to cooperate with the formalirregularities (310) in the form of a pin exceed by at least about 20 to30 mm the dimensions of said formal irregularities (310) at least alongthe axis of rotation (d3) of the moving baseplate (3) so as to enablethere to be an angular clearance of the moving baseplate withoutpermitting a complete rotation of same.

In the embodiment represented in FIGS. 10 and 11, the formalirregularities (310) are a cylindrical drilled hole disposed in thefront of the moving baseplate (3) and adapted to cooperate with thecylindrical head of a pin held in a drilled hole disposed on thefixation head, the dimensions of the cylindrical drilled hole beinggreater than those of the head of the pin by at least about 20 to 30 mm,at least along the axis of rotation (d3) of the moving baseplate (3). Inthis fashion, the pin enables the moving baseplate (3) to turn aroundsaid axis (d3) while holding it in the complementary cavity housed inthe wall of the channel (12).

With regard to the bar (2), the inside surface of the channel (12) is ofsufficient dimensions to enable the bar to have a certain clearance (a2)in rotation around one or a plurality of axes not parallel to thelongitudinal axis of the bar or, in particular, perpendicular to thislongitudinal axis.

At its end on the bar side, the clamping screw (41) constitutes a movingelement, called the support head (42), articulated by a ball and socketconnection. The screwing of the clamping screw (41) provokes thepressing of said support head (42) on the flat of the bar (2) throughone moving clamping face (420) of the moveable clamping system. Saidball and socket connection allows a certain clearance of the supporthead (42) relative to the clamping screw (41) in rotation around thecenter of said ball and socket connection.

By a rotation around at least one axis parallel to the axis of rotation(d3) of the moving baseplate (3), the moving support face (420) can thusbe permanently adjusted to the position of the bar and the movingbaseplate.

Said ball and socket connection also enables the support face to remainin contact with the bar without sliding over it, which avoidsdeterioration of the surfaces in contact, assures the blockage, andreduces the risk of residual stresses.

Thus, it can be understood, that the bar can be inserted and blocked indifferent angular positions inside the channel (12), while providing aflat contact surface both with the clamping means and with the wall ofthe channel by means of the moving baseplate (3). Said polyaxial angularclearance thus allows inserting the bar more easily and obtaining aclamping of the bar in its most natural position relative to theimplants, which reduces or eliminates the stresses that could remain inthe device after clamping. Furthermore, the clamping forces concurdirectly with blocking without necessarily countering the rigidity ofthe bar. The reliability of the blocking is thus improved.

In an embodiment represented in FIGS. 1 to 3, the fixation head (11) isadapted to move relative to the osseous anchor (10) according to anarticulation. This enables freedom in rotation around at least two axesnot coincident with the longitudinal axis of the bar.

This articulation is realized by a complementary spherical contactdisposed between the fixation head (11) and the end of the osseousanchor (10) remote from the osseous element, said end being designatedas the rotation head (101). The rotation head comprises a part (1011) inform of a hemispherical portion widening in the direction of thefixation head (11); that is, by moving away from the osseous element.Said hemispherical portion (1011) is retained on the inside of thefixation head (11) by a complementary contact surface in a housingformed in said fixation head and narrowing itself towards the osseousanchor. Said housing communicates with the channel (12) where it is openin its part situated opposite to the clamp. The spherical nature ofthese contact surfaces thus enable a rotation of the fixation head andthe osseous anchor relative to each other, about the center of saidsurface (1011) that has a hemispherical form.

Said rotations enable, in particular, unlimited clearance (a1) of thefixation head relative to the osseous anchor, in rotation around an axis(d11), called the axis of rotation of the head, that is not parallel,indeed not even perpendicular to the longitudinal axis of the bar or ofthe channel and passing through the center of the hemisphere (1011) ofthe rotation head (101).

These rotations also enable a certain clearance (a4) of the fixationhead relative to the osseous anchor, in rotation around an axis that issubstantially perpendicular to the axis (d11) of rotation of the headand passing through the center of the hemisphere (1011) of the rotationhead (101).

In this embodiment, a mobile base (3) similar to that hereinbeforedescribed is borne by the rotation head (101) in a housing formed on theface opposite to the bar.

At the time of attaching the clamp, the clamping screw (41) co-operatesby its threading with the drilling (14) of the fixation head (11) to thehold the bar (2). The bar presses against the moving baseplate (3). Themoving baseplate (3) leans on the rotation head (101), which is retainedby the housing of the fixation head (11). Clamping of these surfacesamong themselves produces an interlocking of the set of these partsrelative to each other.

It is well understood that in this manner an implant is provided, whosefixation head, prior to affixing of the clamp, is moveable relative tothe osseous anchoring part, while being fixed to the bar after assembly.The fixation head (11) can thus be tilted within a pre-determinedclearance relative to the exterior surface of the osseous element andcan pivot freely around an axis extending from said osseous surface.

Once the implant is anchored in the osseous element, it is thus stillpossible to adjust the position of the fixation head in order to enablethe bar to keep or to resume its shape, which reduces the risks ofresidual stresses and permits easy introduction of the bar into diversepositions of this bar and implants. Once the bar is introduced and thewhole device assembled, it is thus possible to fix said positions byvirtue of the clamp. As clamping can be achieved in the most naturalposition of the pin, the clamping forces are concentrated such that theyachieve fixation reliability. In particular, these clamping forces donot risk, or the risk thereof is minimal, introducing residual stressesor movements relative to the position selected by the surgeon.

In one embodiment of this invention, the clamping means (4) need notcomprise a support head (42) on the ball and socket connection, inparticular if the tilting of the fixation head (11) is sufficientlyclose to that of the bar (2) to assure a planar contact between theclamping screw (41) and the flat of the bar.

In the embodiments represented in the figures, the fixation head (11) ofthe implant (1) has a square external section that enables easyimplantation of the implant (1) into a vertebra using a suitable tool.In other respects, the head passing above the channel (12), makes iteasy to subsequently reposition the implant (1) in the vertebra afterthe bar (2) has been introduced into the channel (12).

It should be clear to the specialist in the art that the presentinvention enables embodiments in many specific forms without moving itaway from the field of application of the invention as claimed.Consequently, the present embodiments must be considered illustrative,but can be modified in the field defined by import of the attachedclaims and the invention should not be limited to the details providedabove.dden

1. An osseous anchorage implant adapted to receive and clamp a barhaving a longitudinal axis, the implant comprising: an osseous anchor;and a fixation head, the fixation head comprising a channel comprising alongitudinal axis, openings at each end of the longitudinal axis of thechannel, and an inside surface configured for a rotational clearance ofthe bar about at least two axes of the channel not parallel to thelongitudinal axis of the channel and not parallel to the longitudinalaxis of the bar, including a support wall adapted to support the bar inany orientation within the rotational clearance of the bar independentlyof the orientation of the osseous anchor, a clamp mount, a clamp havinga support head adapted to hold the bar against the support wall, and alateral opening along the longitudinal axis of the channel, the lateralopening flanking the clamp mount and adapted for lateral insertion ofthe bar at a point along the longitudinal axis of the bar.
 2. An implantaccording to claim 1 in which the inside surface of the channel isconfigured for a rotational clearance of the bar about plural axes ofthe channel not parallel to the longitudinal axis of the channel.
 3. Animplant according to claim 1 in which the support wall comprises amoveable baseplate.
 4. An implant according to claim 3 in which thebaseplate comprises a support face adapted for complementary contactwith a surface of the bar.
 5. An implant according to claim 3 in whichthe baseplate has an axis of rotation not parallel to the longitudinalaxis of the channel.
 6. An implant according to claim 5 in which theclamp has a longitudinal axis, and the axis of rotation of the baseplatealso is not parallel to the longitudinal axis of the clamp.
 7. Animplant according to claim 1 in which the support head articulates at anend of the clamp.
 8. An implant according to claim 6 in which thesupport head articulates at an end of the clamp about an axissubstantially parallel to the axis of rotation of the baseplate.
 9. Animplant according to claim 8 in which the support head articulates at aball and socket joint.
 10. An implant according to claim 4 in which thebaseplate further comprises a portion shaped substantially as a portionof a sphere, which spherically shaped portion is disposed in asubstantially complementary housing in the fixation head.
 11. An implantaccording to claim 6 in which the baseplate further comprises a stoplimiting rotation about the axis of rotation.
 12. An implant accordingto claim 11 in which the stop comprises a pin protruding from thebaseplate into a cavity formed in the fixation head.
 13. An implantaccording to claim 11 in which the stop comprises a pin protruding fromthe fixation head into a cavity formed in the baseplate.
 14. An implantaccording to claim 1 in which the clamp comprises a threaded body andthe clamp mount comprises a tapped hole.
 15. An implant according toclaim 1 in which the osseous anchor comprises a hook.
 16. An implantaccording to claim 1 in which the osseous anchor comprises a screw. 17.A implant according to claim 1 in which the inside of surface of thechannel has a shape comprising two head to tail truncated cones withminor baseplates facing each other and joined to each other.
 18. Animplant according to claim 17 in which the head to tail truncated conesare joined to each other by a cylindrical part.
 19. An implant accordingto claim 1 further adapted to receive and clamp a bar having a clampingsurface along at least a portion of the longitudinal axis of the bar, inwhich the support head comprises a contact surface complementary to theclamping surface of the bar.
 20. An implant according to claim 19 inwhich the contact surface of the support head is flat.
 21. An implantaccording to claim 16 in which the fixation head is fixed to the anchorwith the longitudinal axis of the channel substantially perpendicular toa longitudinal axis of the screw.
 22. An implant according to claim 1 inwhich the fixation head is movable relative to the anchor.
 23. Animplant according to claim 22 in which the fixation head is movablerelative to the anchor about two axes not coincident with thelongitudinal axis of the bar.
 24. An osseous anchorage implant adaptedto receive and clamp a bar having a longitudinal axis, the implantcomprising: an osseous anchor comprising a rotation head having a firsthousing; and a fixation head, the fixation head comprising a secondhousing adapted to receive the rotation head; a channel comprising alongitudinal axis, openings at each end of the longitudinal axis of thechannel, and an inside surface configured for a rotational clearance ofthe bar about at least two axes of the channel not parallel to thelongitudinal axis of the channel and not parallel to the longitudinalaxis of the bar, including a baseplate having a support wall adapted tosupport the bar in any orientation within the rotational clearance ofthe bar independently of the orientation of the osseous anchor, thebaseplate adapted to be disposed in the first housing, a clamp mount, aclamp having a support head adapted to hold the bar against the supportwall, and a lateral opening along the longitudinal axis of the channel,the lateral opening flanking the clamp mount and adapted for lateralinsertion of the bar at a point along the longitudinal axis of the bar.25. An osseous anchorage implant adapted to receive and clamp a barhaving a longitudinal axis, the implant comprising: an osseous anchor;and a fixation head, the fixation head comprising a channel comprising alongitudinal axis, openings at each end of the longitudinal axis of thechannel, and an inside surface configured for a rotational clearance ofthe bar about at least two axes of the channel not parallel to thelongitudinal axis of the channel and not parallel to the longitudinalaxis of the bar, including an articulating support wall adapted tosupport the bar in any orientation within the rotational clearance ofthe bar independently of the orientation of the osseous anchor, a clampmount, a clamp having an articulating support head adapted to hold thebar against the support wall, and a lateral opening along thelongitudinal axis of the channel, the lateral opening flanking the clampmount and adapted for lateral insertion of the bar at a point along thelongitudinal axis of the bar.